1. Field of the Invention
This invention relates to computer systems, and more particularly, to methods and apparatus for providing window overlays on an output display.
2. History of the Prior Art
For some time there has been a need to provide a window on a computer output display in which information from a source other than a computer program may be displayed. For example, it is desirable to display video information provided by a source of television signals or from a digital video disk (DVD) in a window on a computer display.
To accomplish this, the prior art initially combined video signals which had been transformed into RGB format with computer graphic information in the frame buffer. Such an approach provides a window in which video images may be displayed; however, any manipulation of the video data can only be accomplished by the time consuming process of accessing and then returning the manipulated data to the frame buffer.
As the amount of video and graphics data being displayed have both increased, the need to access video data in the frame buffer has greatly slowed the display processes. For this reason, the most recent attempts to display video have concentrated on providing what is referred to as a hardware overlay that displays video information separately from the computer graphics information. The two channels of information are ultimately combined beyond the frame buffer at the end of the graphics pipeline just prior to display.
This method of displaying video information in a window on a computer graphics display performs satisfactorily so long as the manipulation of the video information is limited to color space conversion from a video format to a red/green/blue (RGB) computer graphics format and the video image is either enlarged or maintained at the size provided by the video source. However, if the video image is to be reduced from the size of the source, the data transfer rate during window scanout becomes too great for prior art overlay engines to handle.
It is desirable to provide an improved method and circuitry for furnishing video information in a window on a computer output display.